They're different parts of speech.
If you want to see a pattern, here's one that works in British English:
Noun: advice, practice, licence
Verb: advise, practise, license
Fairly logical, isn't it? The nouns are spelled with a 'c', while the verbs are spelled with an 's'.
This restaurant is licensed to sell alcohol
This restaurant has a licence to sell alcohol.
NB American English only follows this rule with regard to advice/advise. AmE uses 'practice' and 'license' for both nouns and verbs.